The present invention relates to the field of solid state lighting, and in particular to an arrangement of one or more LED strings switched synchronously with input switches of a single stage power supply.
Light emitting diodes (LEDs) and in particular high intensity and medium intensity LED strings are rapidly coming into wide use for lighting applications. LEDs with an overall high luminance are useful in a number of applications including backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a matrix display, as well as for general lighting applications.
In a large LCD matrix display, and in large solid state lighting applications, such as street lighting and signage, typically the LEDs are supplied in a plurality of strings of serially connected LEDs, at least in part so that in the event of failure of one string at least some light is still output. The constituent LEDs of each LED string thus share a common current.
LEDs providing high luminance exhibit a range of forward voltage drops, denoted Vf, and their luminance is primarily a function of current. For example, one manufacturer of LEDs suitable for use with a portable computer, such as a notebook computer, indicates that Vf for a particular high luminance white LED ranges from 2.95 volts to 3.65 volts at 20 mA and an LED junction temperature of 25° C., thus exhibiting a variance in Vf of greater than ±10%. Furthermore, the luminance of the LEDs vary as a function of junction temperature and age, typically exhibiting a reduced luminance as a function of current with increasing temperature and increasing age. In order to provide backlight illumination for a portable computer with an LCD matrix display of at least 25 cm measured diagonally, at least 20, and typically in excess of 40, LEDs are required. In order to provide street lighting, in certain applications over 100 LEDs are required.
In order to provide a balanced overall luminance, it is important to control the current of the various LED strings to be approximately equal. In one embodiment a power source is supplied for each LED string, and the voltage of the power source is controlled in a closed loop to ensure that the voltage output of the power source is consonant with the voltage drop of the LED string, however the requirement for a power source for each LED string is quite costly.
In another embodiment, as described in U.S. Patent Application Publication US 2007/0195025 to Korcharz et al, entitled “Voltage Controlled Backlight Driver” and published Aug. 23, 2007, the entire contents of which is incorporated herein by reference, this is accomplished by a controlled dissipative element placed in series with each of the LED strings. In another embodiment, binning is required, in which LEDs are sorted, or binned, based on their electrical and optical characteristics. Thus, in order to operate a plurality of LED strings from a single power source, at a common current, either binning of the LEDs to be within a predetermined range of Vf is required, or a balancing element, such as the dissipative element of the aforementioned patent application, must be supplied to drop the voltage difference between the strings caused by the differing Vf values so as to produce an equal current through each of the LED strings. Either of these solutions adds to cost and/or wasted energy.
U.S. Pat. No. 7,242,147 issued Jul. 10, 2007 to Jin, entitled “Current Sharing Scheme for Multiple CCF Lamp Operation”, the entire contents of which is incorporated herein by reference, is addressed to a balancer, wherein each CCFL is connected to an AC power source lead via a primary transformer winding. The secondary windings are connected in a closed in-phase loop. The balancer requires an alternating current input in order to avoid DC saturation of the transformers, and is thus not suitable for use with LED strings, which operate only on DC.
LED strings present a significantly different load than incandescent lighting, and in particular the current does not vary in step with the input voltage. The power factor of an alternating current (AC) electric power system is defined as the ratio of real power to the apparent power flowing to a load. Real power is the capacity of the circuit to perform work in a particular time, whereas apparent power is a product of the current and voltage of the circuit. Power is lost in the system when the power factor is significantly below unity. A power factor corrector (PFC) may be advantageously utilized to control the power source providing electrical energy to the LED string so as to achieve a power factor approaching unity. A power factor corrector typically comprises an error amplifier and a multiplier arranged to cooperate so as to maintain a high power factor while controlling a power converter so as to converge the input to the error amplifier towards a reference value.
LED strings exhibit a particular voltage to current relationship, wherein for a voltage below a minimum operating voltage no appreciable current flows, and for voltages exceeding the minimum operating voltage the current follows an exponential curve responsive to the voltage. Small changes in voltage thus result in very large changes in current, which may result in extremely large power surges before correction by the slow response time of the PFC control loop.
A two stage power source and driver provides a first stage with PFC and a second stage which advantageously exhibits a fast control loop, capable of preventing such large power surges. Unfortunately, a two stage power source and driver adds expense and may further exhibit a reduced efficiency as compared with a single stage power source and driver. Additionally, in many prior art applications three stages are in effect provided: the PFC stage, the voltage converter stage and the dissipative balancer stage, which all add to cost and losses.